"There is nothing political about her release, there is nothing political about her imprisonment," Businge told the Reuters news agency.

"The president has granted mercy, and under the constitution, he is allowed to do that," he said, adding that she had asked for mercy twice in the past, including last June.

Ingabire returned to Rwanda from exile in the Netherlands in 2010, planning to run for president, but was barred from standing after being accused of genocide denial and was arrested before the vote took place.

In 2012, she was sentenced to 15 years for conspiring to form an armed group to undermine the government and seeking to minimise the 1994 genocide.

Human Rights Watch has called the charge of "genocide denial" politically motivated and the trial was widely criticised.

Andrew Wallis, author and Rwandan political expert, said Ingabire's letter in June, in which she asked for mercy and confessed to making mistakes, gave the government "leeway to release her now".

"Ingabire is very much a forgotten figure," he said. "We never hear her name come up now. For ordinary Rwandans, the issues are very much health and education and moving forward with their lives.

"This is the sign of a confident government. This is the sign of a government that is moving forward on other issues, and Ingabire is not seen as important."

Kagame's government has won international praise for economic progress in Rwanda, but has faced fierce criticism from rights groups, including allegations of suppressing the media and opposition parties.

Kagame has been president of Rwanda for 18 years and has been the de-facto ruler of the country since 1994. Last year, the Rwandan constitution was amended to end two-term limits for presidents, theoretically allowing Kagame to rule until 2034.